Polyomaviruses cause a variety of severe human diseases particularly in immunocompromised individuals. No specific anti-viral treatments or prophylactic approaches exist to target this family of viruses. There are several critical gaps in our current knowledge of the molecular mechanism of viral replication and tumorigenesis. Our long-term goals are to identify how these viruses subvert normal host cellular processes to facilitate viral replication, and how these interactions may result in oncogenesis. Our previous studies revealed an intricate balanced relationship between viral replication and virus-induced host genomic instability. These results lead to our central hypothesis that an activated cellular DNA damage response (DDR) is important for facilitating viral replication and maintaining host genome stability during polyomavirus infection. Towards this hypothesis, we have identified host mismatch repair system and replicating viral DNA as novel factors contributing to DDR activation. We have also discovered that the ability of polyomavirus to cause host genomic DNA damage is linked to its ability to replicate viral DNA. Guided by strong preliminary data, we propose to pursue three Specific Aims to characterize DDR activation mechanism and how the DDR ties together viral replication and host genomic stability: (1) To define the role of host mismatch repair proteins in polyomavirus replication and polyomavirus-induced DDR activation. (2) To determine the viral DNA triggers that activate the DDR upon polyomavirus infection. (3) To elucidate the molecular mechanism by which polyomavirus induces host genome instability. Collectively, our proposed research will broadly impact the field by characterizing the essential roles that the DDR plays in promoting viral replication and maintaining host genome stability. These studies will have the potential to uncover novel molecular mechanisms underlying polyomavirus replication as well as viral oncogenesis. These findings may be extrapolated to other DNA viruses and to our understanding of normal cellular processes.